In commercial motor controllers for motors in the horsepower range of for example 10 to 200 horsepower, there is generally a circuit which turns on a motor space heater when the motor is not running. Such motor space heaters are necessary for preventing moisture condensation in motors used in outdoor, marine, and high humidity environments. Should the motor heater or motor space heater fail in such high moisture content environments, there is rapid deterioration of insulation resistance of the motor coils causing accelerated burn out of the motor.
Space heater failure is in fact one of the most common causes of motor burn out and loss in high humidity, outdoor and marine environments. Motor windings must be rewound with loss of operation time. In conventional motor controllers, the only indication of the operation of the motor space heater is a heater light wired in parallel with the motor heater. This conventional heater light only indicates that power is being supplied to the motor space heater coils and does not indicate failure of the heating coils or more generally failure of electrical continuity through the motor space heater load.
A conventional motor space heater subcircuit 10 of a standard motor controller is illustrated in FIG. 1. The motor space heater 12 receives power on lines 16 from a motor controller circuit, not shown, and the indicator lamp or light 14 is coupled in parallel with the motor heater 12. The parallel coupled space heater 12 and indicator lamp 14 are coupled in series with a normally closed relay 15 of the motor controller which is opened by a relay activator or armature of the motor controller when the motor is operating. Thus, no power is supplied to the motor space heater 12 and heater lamp 14 when the motor is operating When the motor turns off the relay 15 closes and power from the motor controller is supplied to the space heater 12 and indicator light 14 to prevent moisture condensation in the idle motor coils.
The disadvantage of this conventional arrangement is of course that the indicator lamp 14 is on as long as power is supplied from the motor controller to the motor space heater whether or not the motor space heater 12 is operating. There is no indication provided of failure of the motor heater or failure of electrical continuity through the motor heater coils.